Conventionally known are controllers comprising a valve which is closed by biasing a valve stem downward with a spring and which is opened by driving the valve stem upward with a force given by pneumatic pressure, solenoid or the like and greater than the force of the spring, or those comprising a valve which is opened by biasing a valve stem upward with a spring and which is closed by biasing the valve stem downward with a force afforded by pneumatic pressure, solenoid or the like and greater than the force of the spring.
In the case where such a controller is used for a high-pressure fluid, there is a need to close the valve with an increased force so as to prevent the fluid from leaking. When the controller is of the type wherein a valve is closed by biasing a valve stem downward with a spring and opened by driving the valve stem upward with a force of pneumatic pressure, solenoid or the like, an increase in the elastic force of the spring entails a need to correspondingly increase the pneumatic pressure or like drive force for moving the valve stem. However, the controller has the problem that the valve closing force can not be increased as required because there is a limit on the increase in the drive force. With the controller wherein a valve is opened by biasing a valve stem upward with a spring and closed by biasing the valve stem downward with a force afforded by pneumatic pressure, solenoid or the like and greater than the force of the spring, an increase in the valve closing force entails requires an increase in the pneumatic or like downwardly biasing force. The controller of the latter type therefore has the same problem as described above.
To overcome the above problems, another controller has been proposed which comprises a valve body having a fluid channel to be opened and closed with a reciprocating upward and downward movement of a valve stem, a casing fixed to an upper portion of the valve body, an operating rod provided in an upper inside portion of the casing and movable upward and downward, drive means for moving the operating rod upward and downward, and force amplifying means provided in a lower inside portion of the casing for transmitting a force acting on the operating rod to the valve stem upon amplification (see, for example, the publication of JP-A No. 7-139648). The force amplifying means of this publication comprises a tapered roller contact member extending vertically downward from the lower end of the operating rod, a disklike roller contact member provided at the upper end of the valve stem, a pair of roller supports arranged between the two roller contact members symmetrically about the axis of the tapered member, a pair of rollable rollers rotatably mounted on upper portions of the respective roller supports and bearing against the tapered faces of the tapered roller contact member, and a pair of push rollers rotatably supported by lower portions of the respective roller supports and bearing on the upper surface of the disklike roller contact member. Each of the roller supports is supported by the casing so as to be pivotally movable about a pivot positioned closer to the axis of the tapered roller contact member than the axis of the corresponding push roller.
With the controller disclosed in the above publication, the valve closing force can be increased without increasing pneumatic pressure, the force of a spring or the drive force of a solenoid or the like, so that the controller has the advantage that a high-pressure fluid, even if used, can be reliably prevented from leaking, whereas the controller is complex in construction, comprises an increased number of components and therefore has the problem of being costly to fabricate.
An object of the present invention is to provide a controller which comprises force amplifying means for transmitting a force acting on an operating rod to a valve stem upon amplification and which is simple in construction and reduced in cost.